1. Field of the Invention
This invention relates to wireless communication systems and in particular to a method and apparatus for improved accuracy in equalizing transmit and receive power levels in a base station transceiver to reduce distortion effects experienced on multiple receive and transmit channels.
2. Description of Relevant Art
The ever increasing need for wireless communication services such as Cellular Mobile Telephone (CMT), Digital Cellular Network (DCN), Personal Communication Services (PCS) and the like, typically require the operators of such systems to serve an increasing number of users in a given service area. As a result, certain types of base station equipment, including high capacity broadband transceiver systems (BTS), have been developed which are intended to service a relatively large number of active mobile stations in each cell.
In a broadband transceiver system (BTS), such as the current AdaptaCell BTS produced by AirNet Communications Corporation of Melbourne, Fla., multiple RF channels are transmitted and received concurrently. The frequency response, or gain, for each of these multiple RF channels received by the BTS differ as a result of system imperfections, such as passband ripple or filter roll-off, caused by the components of an RF transceiver in the BTS. The gain difference between these RF channels can be as large as plus or minus 3 dB. This is not a problem for BTS systems employing single channel radio transmitters and receivers rather than broadband transceivers, because single channel radios allow for the gain of each RF channel transmitted and received by the BTS to be set individually, rather than a single gain being added universally across all RF channels in the BTS.
It is desirable to have the gain be equivalent from RF channel to RF channel, because the GSM cellular communications protocol, among others, requires in its specifications that the receive side of a given BTS calculate a specific receive signal strength level with only plus or minus 2 dB of variation. When there is a large amount of variation in the receive signal of an RF carrier, it is difficult to determine the Receive Signal Strength Indication (RSSI) of a mobile station transmitting on the RF channel to the BTS. Without knowledge of the RSSI, it is difficult for the BTS to maintain uplink power control of the mobile station. A loss in uplink power control can result in the loss of signals transmitted by the mobile, unnecessary gain control by the BTS, and a premature expenditure of mobile battery power. In addition to maintaining receive power levels, there are requirements for the BTS to maintain a specific transmit power level across the RF channels of the transceiver as well. The transmit power level must also be maintained in order to prevent the expansion or shrinking of the cell size of the BTS, and for efficiency in the use of transmit power.
The traditional solution to this problem of non-uniform frequency response across RF channels is the use of multiple narrowband single carrier radios to cover the spectrum of RF channels received and transmitted by the BTS, setting the gain on each radio individually for the respective RF channel in a manner in which all RF channels have a uniform frequency response. Another solution that has been used in broadband systems to normalize this frequency response across RF channels has been to take a sampling of average roll-off and ripple values among a cross-section of transceivers. These average roll-off and ripple values were then used to set up a static power table across the transmission range. The static power table did not change for individual transceivers, and was used to artificially flatten the transmit and receive power levels for every transceiver in a BTS on the basis of the average filter roll-off and passband ripple values contained therein. Neither of these solutions provide optimal use of a broadband transceiver having uniformly flat (or normalized) transmit and receive power frequency responses in a given BTS. Thus, a need exists for a base transceiver system that discretely normalizes on every base transceiver within the BTS the frequency response on a channel-by-channel basis.